1. Field of the Invention
The present invention relates to a combustion state detecting device that detects a combustion state of an internal combustion engine by detection of a change in the quantity of ions which is caused at the time of burning in the internal combustion engine, and more particularly to a combustion state detecting device for an internal combustion engine which is downsized, inexpensive and improved in the accuracy of detection.
2. Description of the Related Art
FIG. 3 is a structural diagram schematically showing a conventional combustion state detecting device for an internal combustion engine, in which power distribution is made by one ignition coil for ignition plugs of two cylinders.
In FIG. 3, an anode of a battery 1 mounted on a vehicle is connected to a lower voltage side of a primary winding 2a of an ignition coil 2. The other end of the primary winding 2a is connected to the ground through a power transistor 3 that interrupts the supply of a primary current.
Two ignition coils 2 are disposed in parallel to each other, individually, in correspondence with a pair of ignition plugs 4a, 4c and a pair of ignition plugs 4b, 4d, and the respective pairs of ignition plugs 4a, 4c and 4b, 4d are connected to both ends of the secondary windings 2b.
High-voltage diodes 5 are connected to one end of the respective ignition plugs 4c and 4d of the respective pairs of ignition plugs 4a, 4c and 4b, 4d, respectively, so as to apply a bias voltage identical in polarity with an ignition polarity to one end of the respective ignition plugs 4c and 4d.
The high-voltage diode 5 is provided for the purpose of protecting an ion current detecting unit 10 from an ignition high voltage which is applied to the ends of the ignition plugs 4a to 4d.
The negative pole sides of the respective secondary windings 2b are connected directly to the ignition plugs 4a and 4b, respectively, whereas the positive pole sides of the respective secondary windings 2b are connected to the ignition plugs 4c and 4d through resistors 6 for bias voltage protection, that is, for discharge current limit, respectively.
In addition, the resistors 6 are connected in parallel with ignition diodes 7 a secondary current direction of which is directed forward, respectively.
The cathodes of the respective high-voltage diodes 5 are connected to nodes between the respective resistors 6 as well as the respective ignition diodes 7 and the ignition plugs 4c, 4d, respectively.
With the above structure, in detection of an ion current, the bias voltage is applied to the ignition plugs 4c and 4d directly from one end of the high-voltage diode 5, and the bias voltage is applied to the ignition plugs 4a and 4b through the discharge current limit resistors 6 and the secondary windings 2b.
The ion current detecting unit 10 includes a rectifier diode D1 connected to the other ends of the primary windings 2a, a resistor R1 for current limit which is connected in series to the rectifier diode D1, a Zener diode DZ for voltage limit which is connected in series to the resistor R1, a rectifier diode D2 inserted between the Zener diode DZ and the ground, a capacitor C connected between both ends of the Zener diode DZ so as to be in parallel with the latter, and an output resistor R2 connected in parallel with the rectifier diode D2.
A series circuit consisting of the rectifier diode D1, the resistor R1, the capacitor C and the rectifier diode D2 is disposed between one end of the respective primary windings 2a and the ground so as to constitute a charging path into which a charge current flows to charge the capacitor C.
During the off state of the power transistor 3, the capacitor C is applied with a primary voltage which is a high voltage developed at the primary windings 2a, and charged up to a given bias voltage (about several hundreds V) by the limit voltage of the Zener diode DZ so as to function as a power supply (bias means) for detecting an ion current i. In other words, the capacitor C is charged up to an avalanche voltage of the Zener diode DZ by the primary voltage developed at the time of interrupting the primary current, to thereby ensure a bias voltage necessary for supplying the ion current.
The output resistor R2 within the ion current detecting unit 10 converts the ion current i into a voltage and inputs the voltage thus converted to an ECU 20 as an ion current detection signal Ei.
The ECU 20 made up of a microcomputer judges a combustion state of the internal combustion engine on the basis of the ion current detection signal Ei, and if the ECU 20 detects the deterioration of the combustion state, it appropriately conducts adaptive control.
Also, the ECU 20 arithmetically operates an ignition timing, etc., on the basis of drive conditions obtained from a variety of sensors (not shown), and outputs not only an ignition signal P to the power transistor 3 but also a fuel injection signal to an injector (not shown) for each cylinder, and drive signals to a variety of actuators (a throttle valve, an ISC valve, etc.).
In FIG. 3, a description will be given while attention is paid to only the paired ignition plugs 4a and 4c. The secondary current during the normal ignition control flows in a path that passes through the ignition plug 4a, the secondary winding 2b, the ignition diode 7 and the ignition plug 4c. Conversely, the ignition plugs 4a and 4c are applied with ignition high voltages reverse in polarity to each other.
On the other hand, during detection of the ion current immediately after the ignition control, the ion current i flows through only the ignition plug of a cylinder which has actually conducted an explosion stroke.
In this situation, since the discharge current limit resistor 6 is provided between the high-voltage diode 5 and one end of the secondary winding 2b, the bias voltage can be restrained from being discharged to the ignition coil 2 side at the time of starting the supply of the primary current.
In this example, in case of the circuit shown in FIG. 3, at the time of detecting the ion current, for example, the ignition plug 4c is applied with the bias voltage directly from one end of the high-voltage diode 5, whereas the ignition plug 4a is applied with the bias voltage through the discharge current limit resistor 6 and the secondary winding 2b.
With the above operation, an impedance of the ion current path associated with the ignition plug 4a in the above situation is larger than an impedance of the ion current path associated with the ignition plug 4c by an amount caused by the intervention of the resistor 6 and the secondary winding 2b. Accordingly, assuming that an ion current flows into the ignition plug 4c as indicated by a solid line a in FIG. 4A, an ion current smaller than the current flowing into the ignition plug 4c flows into the ignition plug 4a as indicated by a broken line b in FIG. 4A, with the result that there occurs a difference in ion current between those ignition plugs 4a and 4c.
In addition, a discharge current that flows when the charges in the capacitor C which has been positively charged are discharged to a floating capacitor Cs such as the secondary winding 2b, etc., which have been negatively charged flows as indicated by a solid line c in FIG. 4A.
When the resistance of the resistor 6 is small, the discharge current vibrates to make it difficult to attenuate as indicated by a solid line d in FIG. 4B, with the result that the discharge current is superimposed on the waveform of the ion current as noises.
In the conventional combustion state detecting device for an internal combustion engine, there is provided the ignition current path made up of the ignition plug, the secondary winding and the ignition plug, that is, the discharge current limit resistor and the ignition diode connected in parallel within the secondary current path as described above, and because those parts are required to withstand a voltage developed at the secondary winding when the primary current starts to flow, they must have a peak inverse voltage of about several kV or more, resulting in such a problem that the conventional combustion state detecting device becomes expensive.
Also, an insulation distance between the terminals of the parts per se needs to be elongated for obtaining high withstand voltage, as a result of which not downsized surface installed parts but large-sized lead parts are used as those parts, thereby leading to such a problem that the number of assembling processes increases to make the device expensive.
Further, there arises such a problem that a difference in ion current occurs between a pair of ignition plugs, and additionally the current flowing when the charges in the capacitor which has been positively charged are discharged to a floating capacitor such as the secondary winding, etc., which have been negatively charged vibrates with the result that the current is superimposed on the waveform of the ion current which has been discharged as noises.